In oil well drilling operations, drilling fluid is circulated downwardly through the drill string to wash cuttings away from the drill bit. A mixture of drilling fluid and cuttings is returned to the surface through an annulus. "Adherent drilling fluid" is defined as drilling fluid adhering to the drill cuttings, and, if the drilling fluid is oil-based, the adherent drilling fluid includes oil.
It is well known that the drill cuttings must be separated from the drilling fluid so that the drilling fluid can be recirculated. Additionally, solid cuttings generated in a drilling process, such as during the exploration for oil and gas, which have been contaminated with adherent drilling fluid must be cleansed to remove surface contaminates prior to discharge of the cuttings to the environment. Such apparatuses are particularly beneficial in the laundering or cleansing of drill cuttings on offshore drilling platforms so that the drill cuttings are environmentally safe for discharge to the sea.
One known apparatus utilizes two separate cells each having a low speed agitator to stir a mixture of drill cuttings and cleansing solution. Initially, the drill cuttings are discharged into the first cell containing a surfactant solution and are slowly agitated to mix the surfactant and the drill cuttings together so as to wash the adherent drilling fluid from the drill cuttings. An underflow from this first cell communicates a slurry of surfactant rich with drill cuttings to a second cell containing a solution of surfactant where it is again slowly agitated and further adherent drilling fluid is washed from the drilling cuttings. Thereafter, a slurry of further cleansed drilling cuttings and surfactant is pumped from the second cell to a screening chamber, whereby the surfactant is returned to the system and the cleansed drill cuttings are discharged. Additionally, a portion of the surfactant solution rich in fine drill cuttings and adherent drilling fluids is run through a hydrocyclone separator which discharges the fine drill cuttings along with the larger cleansed drill cuttings.
Such apparatus is believed to lose a portion of the surfactant solution during operation. For example, it is believed such apparatus, which may have hydrocyclone separators with a capacity of one hundred gallons per minute, will lose approximately 4 gallons per minute or 560 gallons per day of surfactant solution. In addition to being environmentally undesirable to discharge this surfactant, the loss of this amount of surfactant is extremely expensive.